The present invention relates to an exposure head, and an image forming apparatus using the same and, for example, to an exposure head in which organic EL light emitting elements as light emitting parts are aligned into an array configuration on a transparent substrate and an image forming apparatus which is miniaturized by employing the exposure head.
In conventional image forming apparatus, such as copying machines, printers, and facsimile machines, utilizing electrophotographic technology, it is common practice to employ a laser scanning optical system as light writing means (exposure means). Under such circumstances, an exposure means has been proposed in Japanese Patent Unexamined Publication No. H11-138899, which employs a single chip on which organic EL light emitting elements are integrated, thereby eliminating variation in light emitting characteristics and reducing the cost. In Japanese Patent Unexamined Publication No. 2000-238333, an exposure head has been proposed in which organic EL light emitting elements and light quantity sensors for measuring the quantity of emitted light are aligned into an array configuration on a same substrate, thereby preventing unevenness of density due to reduction in the quantity of emitted light.
As shown and described in the aforementioned prior publications, light beam emitted from a light emitting part the organic EL light emitting element is incident on a transparent substrate so that projected light is projected from a surface opposite to the surface on which the light emitting part is disposed. As shown in FIG. 16, when light beams are projected from the light emitting part 63 through the transparent substrate 62, light beams from the light emitting part 63 fall into light beams “a” projected from the surface 102 and light beams reflected at the surface 102. The light beams reflected at the projection side surface 102 further fall into light beams “b”, which are incident on the projection side surface 102 at an angle greater than a critical angle “θC” and are thus totally reflected, and light beams “c”, which are reflected at an angle less than or equal to the critical angle “θC”. The reflectance of totally internally reflected light is 100%. Since the reflectance of light reflected at an angle less than the critical angle “θC” is 10% or less, the intensity of light beam “c” is significantly smaller than the intensity of the light beam “b”. After reflected on the projection side surface 102, light beams “b” are incident on the opposite surface 101 on which the light emitting part 63 is disposed. If the opposite surface 101 and the projection side surface 102 are parallel to each other, the light beams are also incident on the opposite surface 101 at an angle greater than the critical angle “θC” and are thus totally internally reflected. In this manner, most of the light beams “b” are finally projected through the end faces of the transparent substrate 62 after being repeatedly totally reflected between the both surfaces 101 and 102 of the transparent substrate 62.
In case of Japanese Patent Unexamined Publication No. 2000-238333 as a prior art, since the light quantity sensors for measuring the quantity of light emitted from light emitting parts are disposed on the same surface on which the light emitting parts are disposed, detectable light beams are only light beams “c”. Therefore, since the detected quantity of light may be poor, highly accurate detection is impossible. Accordingly, there is a problem that it is impossible to prevent unevenness of density with a high degree of accuracy.